Fast imaging plays an important role in an ultrasonic diagnostic system in that it helps to provide a basis for more advanced techniques, i.e. a high data rate. With more information collected within a unit time, it becomes possible to perform finer image analysis and implement various techniques well. In particular, its advantages are as follows:
1. Improvement on 3D/4D Imaging
Both 3D and 4D imaging are on the basis of processing an extremely large amount of data. The imaging rate may be limited by the relatively low frame rate, which in turn poses limitations on 3D image speed.
2. Improvement on Blood Flow Imaging
The frame rate and image quality of blood flow imaging both have a direct influence on the interaction between a user and the system, which is an important criterion on the system level. In some systems, the frame rate of flow imaging is under that of C mode in some advanced systems. Thus the fast imaging is very important. To make it simple, the principle of fast imaging lies in generating the data of a plurality of scan-lines with the data received during one transmission, which helps to obtain the data of scan-lines in parallel. As a result, the frame rate of flow imaging can be increased significantly.
3. Improvement on Cardiac Imaging
For a heart moving fast, the frame rate is sometimes even more important than image quality.
4. Improvement on Image Quality
Many prior arts may be summed up as coming under different tradeoffs between image quality and frame rate, e.g.:
i) Synthesis aperture uses two transmissions to synthesize one scan-line with high signal to noise ratio.
ii) Composite imaging improves the image quality and reduces speckles by repeatedly transmitting composite scan-lines at different angles.
iii) It takes several transmissions to reduce the effect of longitudinal side lobes during the transmission of Golay code in coded excitation.
iv) In B mode imaging of heart, high frame rate is achieved by using low-density scan.
In the above-described examples, i)-iii) improve image quality at the expenses of frame rate, while iv) increases frame rate at the expenses of image quality. This contradiction between image quality and frame rate can be relieved by the fast imaging, which helps to implement these techniques in a better way.
5. Improvement on Heart-Related Techniques
In prior arts, many advanced systems are related to clinical techniques of heart, such as anatomic M-mode and the related analysis on the movement of heart. These techniques use the variation over time of the location of a certain part of heart in the image to conduct clinical evaluation and indicator calculation, in order to get a continuous image and a precise result. This imposes a critical requirement on the time resolution of the heart image, while the time resolution is actually the frame rate of image.
Multi-beam reception has become the focus of study for the purpose of improving frame rate. In multi-beam reception, a plurality of scan-lines is received during one transmission so as to reduce the time needed to generate one image frame, and thereby improve the frame rate significantly. A main problem associated with this technique is distortion, i.e., obtained scan-lines will be distorted if the transmit beam does not cover all the scan-lines. Therefore, an important issue concerned with multi-beam techniques is to make the transmit beam to cover a range of received scan-lines, i.e., a technique for transmission of wide beam.
A method of transmission of wide beam is disclosed in U.S. Pat. No. 6,585,648, titled “System, method and machine readable program for performing ultrasonic fat beam transmission and multilane receive imaging”, wherein the transmit waveforms of a plurality of transmissions are accumulated to obtain a wide transmit beam. According to this patent, for a single beam, the transmission of each beam corresponds to different delayed transmit waveforms of a plurality of array elements; for various scan-lines, delays of transmit waveforms are different from each other. As a result, incorporating a plurality of transmission into one will accumulate a plurality of transmit waveforms of one array element and a composite waveform of the wide beam of the array element is obtained. Since the delays are different from each other, the transmit waveforms obtained at each array element are different from each other as well. The transmission result of such a waveform can be actually regarded as an accumulation of acoustic fields generated by single beam transmissions, thereby a wide beam is obtained.
A method for optimization of ultrasonic beam is disclosed in U.S. Pat. No. 6,282,963, titled “Numerical optimization of ultrasound beam path”. The main principle of the patent is to obtain a wide beam by optimizing the transmit apodizing curves. This method mathematically models the transmit beam (taking into account the effect of the apodizing curves) and proposes criterions on evaluation of wide beam. An optimized transmit apodizing curve is obtained by optimizing math equations.
The above-discussed techniques have the disadvantages that wide beam is obtained by transmitting any waveform (U.S. Pat. No. 6,585,648) or by controlling the transmit apodizing curve (U.S. Pat. No. 6,282,963). However, the premise for implementing these methods is that a front end of an ultrasonic system can transmit arbitrary waveform, which is impossible for lots of ultrasonic systems that can transmit only excitation waveform of unipolar or bipolar levels.